Serela isn't kidding when she says that there's also some passives that level up REALLY LATE. Like 65-70.
What do I mean? The -90% hp penalty passive that the metal king yukkuri shikigami has? At 65, it goes down to -45%, which is why not all passives at certain levels have been filled in yet.
In the end, this means some shikigamis might have some absurdly powerful passives that aren't known yet because they require level 60-70 to get. Unfortunately, the best exp rate gain is in the last area before the end via the highest tier yukkuris which give 30k exp each, so it'll take a good while to get all the high high level passives. At least you can actually equip 15 shikigamis, but due to the way that hunting highest tier yukkuris works, it's still limited in how many shikigamis you can actually level up, though.